The combustion stoichiometry of methane gas at 1000° F. is highly exothermic and produces carbon dioxide and water according to the following reaction:CH4+2O2→CO2+2H2O (−190.3 kcal/g mol CH4).The formed gases are not useful for the production of valuable chemical compounds, and the stability of these products complicates their conversion to more desirable products. Also, further processing is problematic due to the high temperatures generated in the combustion reaction, presenting problems with respect to downstream reactors and catalysts.
In contrast, useful gases, known as synthesis gas or “syngas”, are produced in the conversion of methane to a gas mixture containing hydrogen and carbon monoxide, for example, according to one of the following reactions:2CH4+2O2→2CO+2H2+2H2O (−64 kcal/g mol CH4)2CH4+1.5O2→2CO+3H2+H2O (−34.9 kcal/g mol CH4), or2CH4+O2→2CO+H2 (−5.7 kcal/g mol CH4).The last reaction is the most desirable in terms of both quality of the syngas produced and the minimization of the heat liberated to protect the apparatus and catalyst bed from thermal damage.
Conventional syngas-generating processes include steam methane reforming and methane partial oxidation (also referred to as catalytic partial oxidation). The choice of a particular route depends primarily on the desired product composition, as determined by its end use. The syngas is typically used to produce methanol, ammonia, or heavier hydrocarbon fuels through Fisher-Tropsch technology for example.
In the steam methane reforming process, the methane-containing and oxygen-containing feeds are mixed and reacted in a diffusion flame. The oxidized effluent is then passed into a steam reforming reactor zone where the effluent is contacted with a conventional steam reforming catalyst. The catalyst may be present as a simple fixed bed or impregnated into a monolith carrier or ceramic foam. The high temperatures and pressures in the catalytic steam reforming reactor zone place great demands on the reactor, in terms of thermal resistance to damage and operational cost, and on the reforming catalyst, in terms of the catalyst's ability to substantially retain its catalytic activity and stability over many years of use due to substantial coking of the catalyst. Moreover, some of the methane may be burned to provide heat for the energy intensive process and therefore, complete conversion of the methane to the desired end product is not achieved thereby reducing the overall selectivity of the process.
In the methane partial oxidation process, methane is passed along a nickel-based catalyst for example, and burned at a high temperature in an oxygen lean environment where the methane is partially oxidized to carbon monoxide along with the production of hydrogen and some steam. The methane partial oxidation process uses a higher concentration of oxygen than is found in air and, therefore, requires the use of a costly air separation unit to supply the higher concentration of oxygen. Moreover, as in the steam reforming process, the methane partial oxidation process typically burns a portion of the methane to provide heat for the energy intensive process and therefore, complete conversion of the methane to the desired end product is not achieved thereby reducing the overall selectivity of the process. Furthermore, the methane partial oxidation process results in a substantial amount of coke being deposited onto the catalyst and therefore, catalyst activity and stability over time is substantially reduced.
Accordingly, it is desirable to provide processes and systems that provide relatively high conversion of methane to syngas, preferably without depositing substantial amounts of coke onto the catalyst so that the catalyst better maintains its activity and stability over time, and which are robust and cost-effective. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention in the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.